The present invention relates to a continuous process for on line gas phase coating of polymerisation catalyst. The present invention also relates to a continuous gas phase fluidised bed process for the production of polyolefins having improved levels of productivity without fouling, more particularly of polyethylene, comprising the introduction of a coated polymerisation catalyst obtained by the continuous gas phase coating process according to the present invention.
In the fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed comprises a bed of growing polymer particles, polymer product particles and catalyst particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas which comprises recycle gas from the top of the reactor together with make-up feed. The fluidising gas enters the bottom of the reactor and is passed, preferably through a fluidisation grid, to the fluidised bed.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomeric olefin, optionally together with, for example, an inert diluent gas such as nitrogen and/or lower alkanes such as ethane, propane, butane, pentane, hexane, and/or a gaseous chain transfer agent such as hydrogen. Thus the recycle gas serves to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas to the recycle gas stream.
It is well known that the production rate (i.e. the space time yield in terms of weight of polymer produced per unit volume of reactor space per unit time) in commercial gas fluidised bed reactors of the above-mentioned type is restricted by the maximum rate at which heat can be removed from the reactor. The rate of heat removal can be increased for example, by increasing the velocity of the recycle gas and/or reducing the temperature of the recycle gas and/or changing the heat capacity of the recycle gas. However, there is a limit to the velocity of the recycle gas which can be used in commercial practice. Beyond this limit the bed can become unstable or even lift out of the reactor in the gas stream, leading to blockage of the recycle line and damage to the recycle gas compressor or blower. There is also a limit on the extent to which the recycle gas can be cooled in practice. This is primarily determined by economic considerations, and in practise is normally determined by the temperature of the industrial cooling water available on site. Refrigeration can be employed if desired, but this adds to the production costs. Thus, in commercial practice, the use of cooled recycle gas as the sole means of removing the heat of polymerisation from the gas fluidised bed polymerisation of olefins has the disadvantage of limiting the maximum production rates obtainable.
The prior art suggests a number of methods for increasing the heat removal capacity of the recycle stream, such as U.S. Pat. No. 4,543,399, U.S. Pat. No. 4,588,790, U.S. Pat. No. 5,352,749, xc2x13 U.S. Pat. No. 5,436,304, U.S. Pat. No. 5,453,471 and U.S. Pat. No. 5,541,270, the contents of which are hereby incorporated by reference.
The above-disclosed processes have all contributed to increase the levels of productivity which are achievable in fluid bed polymerisation processes, which is also one of the objectives according to the present invention. It is known however in the art that a major problem encountered in those high productivity polymerisation processes is the fouling phenomena which can occur at any time in the reactor.
One of the main problems encountered in the fluid bed processes for the production of polyethylene and ethylene copolymers is the reactor fouling, as usually referenced in the literature. The use of catalytic system presenting an increasingly high activity, especially at polymerisation start-up, tends also to have a detrimental impact on this fouling phenomenon. Today, those problems are further exacerbated at the industrial scale where the production capacity of polymerisation reactors tend to increase, e.g. like for industrial ethylene fluidised bed polymerisation where more than 350 Mkg of polyethylene per year can be achieved in a single reactor.
The impact of fouling or agglomerates is very high since agglomerates may grow quite large before coming loose and falling into the fluid bed. Once fallen into the main fluid bed, they can obstruct powder fluidisation, circulation, and withdrawal. When powder withdrawal slows or the bed fuses, the reactor production must be stopped and the reactor vessel opened for cleaning. This is a very costly production outage.
There are a lot of disclosures in prior art of those fouling phenomena as well as many different tentative explanations for its occurrence. Sometimes the type of catalyst used is said to be responsible for the fouling; static electricity has also been indicated as being a cause thereof; operating conditions have also been considered as the most important criteria; in fact, the man in the art has developed many different theories and proposals for explaining and trying to reduce fouling phenomena. It would be a major improvement in the art if all these fouling phenomena could be either considerably reduced or eliminated whatever the explanation of their occurrence.
The Applicants have now unexpectedly found that the fouling problems usually encountered in the above-disclosed prior art process can be considerably reduced or even eliminated when using the process according to the present invention.
We have now found a process which is easy to implement, which could be applied with all types of polymerisation catalysts, which considerably reduces or even eliminates the potential fouling phenomena inside the reactor, and which further brings many other advantages as will be apparent from the present disclosure.
The present invention provides a new continuous process allowing an improved on line gas phase coating of polymerisation catalyst.
Different prior art documents described the coating of polymerisation catalysts.
EP-622382 discloses a propylene/ethylene copolymerisation using a coated catalyst obtained by treating with one monomer a mixture of a conventional supported heterogeneous Ziegler-Natta catalyst component, an organo-Al cocatalyst and an electron-donor. The coated catalyst has a polymer coating:catalyst weight ratio below 10:1. The use of the ex-situ or in-situ produced coated catalyst gives an increase in randomness without the need for other process or catalyst system changes.
EP-588277 discloses a continuous olefin polymerisation process comprising the addition of a coated catalyst wherein the catalyst has a polymeric coating in a weight ratio of coating:catalyst of less than 10:1.
EP-338676 discloses a Ziegler-Natta type catalyst for (co)polymerisation of propylene in the form of a pre-activated support. The pre-activated support is treated with TiCl4; the treated support is placed in contact with an alkyl aluminium halide and propylene, optionally mixed with ethylene and/or 4-8C alpha-olefin to form a coated catalyst containing 0.1 to 10 g of propylene (co)polymer per mole of Ti.
Those prior art documents disclose many different advantages resulting from the use of the so prepared coated catalysts. However, the Applicants have found that the use of those prior art coated catalysts does not allow to overcome the primary fouling concern, as discussed hereabove.
Thus, according to the present invention, there is provided a continuous process for gas phase coating of polymerisation catalyst characterised in that the polymerisation catalyst is introduced in a gas phase plug flow type reactor wherein it is submitted to polymerisation conditions in the presence of at least one monomer such that at least 95% by weight of the produced coated catalysts have a coating yield comprised between 0.5 to 2 times the average coating yield.